Stacking Faults and Transformation of .GAMMA.' Metastable Precipitates in an Fe-29Ni-22Co-4Nb-2Cr-1Ti-0.5Al-0.5Si Alloy

Abstract

In this study, we investigate an Fe-29Ni-22Co-4Nb-2Cr-1Ti-0.5AI-0.5Si heat resistant alloy (refer to herein as alloy 929C), in which the y' phase is the precipitation-strengthening phase. The specimens of alloy 929C are solid-solution heat treated and aged within a temperature range of 993 to 1 073 K for up to 1 440 ks. The morphological and structural changes of the precipitates in the alloy are analyzed by transmission electron microscopic observation. Internal-fringe contrast, which suggests the existence of stacking faults on the {111}, plane, is found in many of the large γ' precipitates formed in the specimens at the latter stage of aging at temperatures above 1 033 K. The metastable y' precipitates, of which some have stacking faults, are gradually transformed into a stable r] phase during aging. The effects of stacking faults introduced by cold-rolling into the y' particles on the formation of η phase are studied by subsequent annealing heat treatments. The selected-area electron diffraction (SAED) patterns of the cold-rolled and annealed particles show that the metastable y' precipitates with stacking faults are transformed intensively into a stable η phase. In this paper, we discuss in detail the basis of these morphological and structural changes of the precipitates in heat-resistant alloys.